BME 301: Biomedical Sensors

Transcription

1 BME 301: Biomedical Sensors Lecture Note 2: Sensor Characteristics, Transducer Systems, Displacement and Temperature Sensors 1 BME 301 Lecture Note 2 - Ali Işın 2017

2 Sensor is a Transducer: What is a transducer? A device which converts one form of energy to another Sensors Physical parameter Actuators Electrical Input e.g. Piezoelectric: Force -> voltage Voltage-> Force Electrical Output Physical Output 2 BME 301 Lecture Note 2 - Ali Işın 2017

3 Performance Characteristics 1/3 Transfer Function: The functional relationship between physical input signal and electrical output signal. Usually, this relationship is represented as a graph showing the relationship between the input and output signal, and the details of this relationship may constitute a complete description of the sensor characteristics. Sensitivity: Relationship between input physical signal and output electrical signal. The ratio between a small change in electrical signal to a small change in physical signal. As such, it may be expressed as the derivative of the transfer function with respect to physical signal. Typical units : Volts/Kelvin. A Thermometer would have "high sensitivity" if a small temperature change resulted in a large voltage change. Span or Dynamic Range: The range of input physical signals which may be converted to electrical signals by the sensor. Signals outside of this range are expected to cause unacceptably large inaccuracy. This span or dynamic range is usually specified by the sensor supplier as the range over which other performance characteristics described in the data sheets are expected to apply. 3 BME 301 Lecture Note 2 - Ali Işın 2017

4 Performance 2/3 Precise Accurate Accuracy: Generally defined as the largest expected error between actual and ideal output signals. Sometimes this is quoted as a fraction of the full scale output. For example, a thermometer might be guaranteed accurate to within 5% of FSO (Full Scale Output) Precision: How repeatable the sensor is. Uniformity in reproducing one result. Depends on method. Here there is no comparison to a standard, to a particular ideal result. Hysteresis: Some sensors do not return to the same output value when the input stimulus is cycled up or down. The width of the expected error in terms of the measured quantity is defined as the hysteresis. Typical units: Kelvin (for temperature sensors) or % of FSO Nonlinearity (often called Linearity): The maximum deviation from a linear transfer function over the specified dynamic range. There are several measures of this error. The most common compares the actual transfer function with the best straight line', which lies midway between the two parallel lines which encompasses the entire transfer function over the specified dynamic range of the device. This choice of comparison method is popular because it makes most sensors look the best. 4 BME 301 Lecture Note 2 - Ali Işın 2017

5 Performance Characteristics 3/3 Noise: All sensors produce some output noise in addition to the output signal. The noise of the sensor limits the performance of the system based on the sensor. Noise is generally distributed across the frequency spectrum. Many common noise sources produce a white noise distribution, which is to say that the spectral noise density is the same at all frequencies. Since there is an inverse relationship between the bandwidth and measurement time, it can be said that the noise decreases with the square root of the measurement time. Resolution: The resolution of a sensor is defined as the minimum detectable signal fluctuation. Since fluctuations are temporal phenomena, there is some relationship between the timescale for the fluctuation and the minimum detectable amplitude. Therefore, the definition of resolution must include some information about the nature of the measurement being carried out. Bandwidth: All sensors have finite response times to an instantaneous change in physical signal. In addition, many sensors have decay times, which would represent the time after a step change in physical signal for the sensor output to decay to its original value. The reciprocal of these times correspond to the upper and lower cutoff frequencies, respectively. The bandwidth of a sensor is the frequency range between these two frequencies. 5 BME 301 Lecture Note 2 - Ali Işın 2017

6 Basic Sensors and Principles 6 BME 301 Lecture Note 2 - Ali Işın 2017

7 Transducer Systems Sensors Actuators Interface Circuits Control and Processing Circuits Power Supply I/O Channel /USER 7 BME 301 Lecture Note 2 - Ali Işın 2017

8 Classification of Transducers Transducers On The Basis of principle Used Active/Passive Primary/Secondary Analogue/Digital Transducers/ Inverse Transducers Capacitive Inductive Resistive Transducers may be classified according to their application, method of energy conversion, nature of the output signal, and so on. 8 BME 301 Lecture Note 2 - Ali Işın 2017

9 SPECIAL REQUIREMENTS OF SENSOR IN BIOMEDICAL APPLICATIONS Appliances for diagnosis: measuring or mapping a parameter at a given time Monitoring devices for measuring parameters within a given period Built-in controlling units containing not only sensors but also actuators 9 BME 301 Lecture Note 2 - Ali Işın 2017

10 Passive Transducers 10 BME 301 Lecture Note 2 - Ali Işın 2017

11 11 BME 301 Lecture Note 2 - Ali Işın 2017

12 Active Transducers 12 BME 301 Lecture Note 2 - Ali Işın 2017

13 Selecting a Transducer What is the physical quantity to be measured? Which transducer principle can best be used to measure this quantity? What accuracy is required for this measurement? Fundamental transducer parameters Physical conditions Environmental conditions Compatibility of the associated equipment Reducing the total measurement error : Using in-place system calibration with corrections performed in the data reduction Artificially controlling the environment to minimize possible errors 13 BME 301 Lecture Note 2 - Ali Işın 2017

14 Transducer, Sensor, and Actuator Transducer: a device that converts energy from one form to another Sensor: converts a physical parameter to an electrical output (a type of transducer, e.g. a microphone) Actuator: converts an electrical signal to a physical output (opposite of a sensor, e.g. a speaker) 14 BME 301 Lecture Note 2 - Ali Işın 2017

15 Type of Sensors Displacement Sensors: resistance, inductance, capacitance, piezoelectric Temperature Sensors: Thermistors, thermocouples Electromagnetic radiation Sensors: Thermal and photon detectors 15 BME 301 Lecture Note 2 - Ali Işın 2017

16 Displacement Measurements Used to measure directly and indirectly the size, shape, and position of the organs. Displacement measurements can be made using sensors designed to exhibit a resistive, inductive, capacitive or piezoelectric change as a function of changes in position. 16 BME 301 Lecture Note 2 - Ali Işın 2017

17 Resistive sensors - potentiometers Measure linear and angular position Resolution a function of the wire construction Measure velocity and acceleration 2 to 500mm From 10 o to more than 50 o 17 BME 301 Lecture Note 2 - Ali Işın 2017

18 Resistive sensors strain gages Devices designed to exhibit a change in resistance as a result of experiencing strain to measure displacement in the order of nanometer. For a simple wire: R L A A change in R will result from a change in (resistively), or a change in L or A (dimension). The gage factor, G, is used to compare various strain-gage materials G R L / / R L 1 2 / L / L Is Poisson s ratio D L / / D L for most metals =0.3 Semiconductor has larger G but more sensitive to temperature 18 BME 301 Lecture Note 2 - Ali Işın 2017

19 19 BME 301 Lecture Note 2 - Ali Işın 2017

20 Wheatstone Bridge v o is zero when the bridge is balanced- that is when R 1 R2 R4 / / R 3 If all resistor has initial value R 0 then if R 1 and R 3 increase by R, and R 2 and R 4 decreases by R, then v 0 R R 0 v i 20 BME 301 Lecture Note 2 - Ali Işın 2017

21 Unbonded strain gage: Fig. legend: R 1 = A, R 2 = B, R 3 = D, R 4 = C Wheatstone Bridge With increasing pressure, the strain on gage pair B and C is increased, while that on gage pair A and D is decreased. A B C D ) )( ( ) ( ) ( R R R R R R R R R R V V R R R R R R V V V V R R R V V R R R V V i o i b a o i b i a Initially before any pressure R 1 = R 4 and R 3 = R 2 21 BME 301 Lecture Note 2 - Ali Işın 2017

22 Bonded strain gage: - Metallic wire, etched foil, vacuum-deposited film or semiconductor is cemented to the strained surface Rugged, cheap, low mass, available in many configurations and sizes To offset temperature use dummy gage wire that is exposed to temperature but not to strain 22 BME 301 Lecture Note 2 - Ali Işın 2017

23 Bonded strain gage terminology: Carrier (substrate + cover) 23 BME 301 Lecture Note 2 - Ali Işın 2017

24 24 BME 301 Lecture Note 2 - Ali Işın 2017

25 Semiconductor Integrated Strain Gages Pressure strain gages sensor with high sensitivity Integrated cantilever-beam force sensor 25 BME 301 Lecture Note 2 - Ali Işın 2017

26 4 cm Clear plastic To patient Saline Flush valve Gel IV tubing Silicon chip Electrical cable Isolation in a disposable blood-pressure sensor. Disposable blood pressure sensors are made of clear plastic so air bubbles are easily seen. Saline flows from an intravenous (IV) bag through the clear IV tubing and the sensor to the patient. This flushes blood out of the tip of the indwelling catheter to prevent clotting. A lever can open or close the flush valve. The silicon chip has a silicon diaphragm with a four-resistor Wheatstone bridge diffused into it. Its electrical connections are protected from the saline by a compliant silicone elastomer gel, which also provides electrical isolation. This prevents electric shock from the sensor to the patient and prevents destructive currents during defibrillation from the patient to the silicon chip. 26 BME 301 Lecture Note 2 - Ali Işın 2017

27 Elastic-Resistance Strain Gages Extensively used in Cardiovascular and respiratory dimensional and volume determinations. As the tube stretches, the diameter decreases and the length increases, causing the resistance to increase b) venous-occlusion plethysmography c) arterial-pulse plethysmography Filled with a conductive fluid (mercury, conductive paste, electrolyte solution. Resistance = /cm, linear within 1% for 10% of maximal extension 27 BME 301 Lecture Note 2 - Ali Işın 2017

28 Inductive Sensors Ampere s Law: flow of electric current will create a magnetic field Faraday s Law: a magnetic field passing through an electric circuit will create a voltage + - v i v N d dt v v N 1 N 2 v N 1 1 v2 N 2 28 BME 301 Lecture Note 2 - Ali Işın 2017

29 Inductive Sensors Ampere s Law: flow of electric current will create a magnetic field Faraday s Law: a magnetic field passing through an electric circuit will create a voltage Self-inductance Mutual inductance Differential transformer n 2 G n = number of turns of coil G = geometric form factor di = effective magnetic permeability of the medium L v L dt 29 BME 301 Lecture Note 2 - Ali Işın 2017

30 LVDT : Linear variable differential transformer - full-scale displacement of 0.1 to 250 mm mv for a displacement of 0.01mm - sensitivity is much higher than that for strain gages Disadvantage requires more complex signal processing v o v cd v ce v de _ - (a) As x moves through the null position, the phase changes 180, while the magnitude of v o is proportional to the magnitude of x. (b) An ordinary rectifierdemodulator cannot distinguish between (a) and (b), so a phase-sensitive 30 BME 301 Lecture Note 2 - Ali Işın 2017 demodulator is required.

31 Capacitive Sensors Capacitive sensors For a parallel plate capacitor: C 0 r A x 0 = dielectric constant of free space r = relative dielectric constant of the insulator A = area of each plate x = distance between plates Change output by changing r (substance flowing between plates), A (slide plates relative to each other), or x. 31 BME 301 Lecture Note 2 - Ali Işın 2017

32 Sensitivity of capacitor sensor, K C x 0 r Sensitivity increases with increasing plate size and decreasing distance A 2 x When the capacitor is stationary x o the voltage v 1 =E. A change in position x = x 1 -x o produces a voltage v o = v 1 E. i + + V X o ( j) ( j) E / xo j 1 j 1 Characteristics of capacitive sensors: High resolution (<0.1 nm) Dynamic ranges up to 300 µm (reduced accuracy at higher displacements) High long term stability (<0.1 nm / 3 hours) Bandwidth: 20 to 3 khz 32 BME 301 Lecture Note 2 - Ali Işın 2017 i dvc c dt

33 Example 2.1 For a 1 cm 2 capacitance sensor, R is 100 MΩ. Calculate x, the plate spacing required to pass sound frequencies above 20 Hz. Answer: From the corner frequency, C =1/2πfR=1/(2π )= 80 pf. x can be calculated as follows: x x A 0 r C ( )( m 1.11μm 4 ) 33 BME 301 Lecture Note 2 - Ali Işın 2017

34 Piezoelectric Sensors Measure physiological displacement and record heart sounds Certain materials generate a voltage when subjected to a mechanical strain, or undergo a change in physical dimensions under an applied voltage. Uses of Piezoelectric External (body surface) and internal (intracardiac) phonocardiography Detection of Korotkoff sounds in blood-pressure measurements Measurements of physiological accelerations Provide an estimate of energy expenditure by measuring acceleration due to human movement. 34 BME 301 Lecture Note 2 - Ali Işın 2017

35 V o q k kf piezoelectric constant,c/n (typically pc/n, a material property) k for Quartz = 2.3 pc/n k for barium titanate = 140 pc/n To find V o, assume system acts like a capacitor (with infinite leak resistance): V o q C kf C kfx 0 A r Capacitor: C 0 r For piezoelectric sensor of 1-cm 2 area and 1-mm thickness with an applied force due to a 10-g weight, the output voltage v is 0.23 mv for quartz crystal 14 mv for barium titanate crystal. A x 35 BME 301 Lecture Note 2 - Ali Işın 2017

36 Models of Piezoelectric Sensors Piezoelectric polymeric films, such as polyvinylidence fluoride (PVDF). Used for uneven surface and for microphone and loudspeakers. 36 BME 301 Lecture Note 2 - Ali Işın 2017

37 Transfer Function of Piezoelectric Sensors View piezoelectric crystal as a charge generator: q Kx K proportionality x deflection constant R s : sensor leakage resistance C s : sensor capacitance C c : cable capacitance C a : amplifier input capacitance R a : amplifier input resistance R a 37 BME 301 Lecture Note 2 - Ali Işın 2017

38 Convert charge generator to current generator: i i s c C Vo X Transfer Function of Piezoelectric q Kx i i c s dv dt i i o R R K i s dx dt j Ks j j j 1 dq dt Vo R K s = K/C, sensitivity, V/m = RC, time constant K Sensors dx dt Current 38 BME 301 Lecture Note 2 - Ali Işın 2017 R a R a

39 Voltage-output response of a piezoelectric sensor to a step displacement x. Decay due to the finite internal resistance of the PZT q VC V 0 Kx C Kx The decay and undershoot can be minimized by increasing the time constant =RC. 39 BME 301 Lecture Note 2 - Ali Işın 2017

40 Example 2.2 A piezoelectric sensor has C = 500 pf. Sensor leakage resistanse is 10 GΩ. The amplifier input impedance is 5 MΩ. What is the low corner frequency? 40 BME 301 Lecture Note 2 - Ali Işın 2017

41 Example 2.2 C = 500 pf R leak = 10 G R a = 5 M What is f c,low? Current 1 2RC 2 (510 1 )( f c, low 6 12 ) 64 Hz If input impedance is increased 100 times: (R a = 500 M ) Then the f c,low : 2 ( )( f c, low 6 12 ) 0.64 Hz 41 BME 301 Lecture Note 2 - Ali Işın 2017

42 High Frequency Equivalent Circuit R s Vo X j Ks j j j 1 42 BME 301 Lecture Note 2 - Ali Işın 2017

43 Temperature Measurement The human body temperature is a good indicator of the health and physiological performance of different parts of the human body. Temperature indicates: Shock by measuring the big-toe temperature Infection by measuring skin temperature Arthritis by measuring temperature at the joint Body temperature during surgery Infant body temperature inside incubators Temperature sensors type Thermocouples Thermistors Radiation and fiber-optic detectors p-n junction semiconductor (2 mv/ o C) 43 BME 301 Lecture Note 2 - Ali Işın 2017

44 Thermocouple Electromotive force (emf) exists across a junction of two dissimilar metals. Two independent effects cause this phenomena: 1- Contact of two unlike metals and the junction temperature (Peltier) T 1 B A B T 2 T 1 2- Temperature gradients along each single conductor (Lord Kelvin) E = f (T T 22 ) Advantages of Thermocouple fast response (=1ms), small size (12 μm diameter), ease of fabrication and long-term stability Disadvantages E = f(t 1 T 2 ) Small output voltage, low sensitivity, need for a reference temperature 44 BME 301 Lecture Note 2 - Ali Işın 2017

45 Thermocouple Empirical calibration data are usually curve-fitted with a power series expansion that yield the Seebeck voltage. T 1 B A B T 2 T 1 E = f(t 1 T 2 ) E at 1 bt T: Temperature in Celsius Reference junction is at 0 o C 45 BME 301 Lecture Note 2 - Ali Işın 2017

46 Thermocouple Laws 1- Homogeneous Circuit law: A circuit composed of a single homogeneous metal, one cannot maintain an electric current by the application of heat alone. See Fig. 2.12b (next slide) 2- Intermediate Metal Law: The net emf in a circuit consisting of an interconnection of a number of unlike metals, maintained at the same temperature, is zero. See Fig. 2.12c (next slide) E 46 - Second law makes it possible for lead wire connections 3- Successive or Intermediate Temperatures Law: See Fig. 2.12d (next slide) 23 The third law makes it possible for calibration curves derived for a given reference-junction temperature to be used to determine the calibration curves for another reference temperature. 1 2 E13 E12 a1t 3 bt 1 3 a1t 2 bt 1 2 T BME 301 Lecture Note 2 - Ali Işın 2017 T 1 T 3

47 47 BME 301 Lecture Note 2 - Ali Işın 2017

48 Thermoelectric Sensitivity For small changes in temperature: E E T Differentiate above equation to find, the Seebeck coefficient, or thermoelectric sensitivity. Generally in the range of V/ o C at 20 o C. at de dt 1 2 a bt 2 bt T 1 A E = f(t 1 T 2 ) B T 2 48 BME 301 Lecture Note 2 - Ali Işın 2017

49 Thermistors Thermistors are semiconductors made of ceramic materials whose resistance decreases as temperature increases. Advantages Small in size (0.5 mm in diameter) Large sensitivity to temperature changes (-3 to -5% / o C) Blood velocity Temperature differences in the same organ Excellent long-term stability characteristics (R=0.2% /year) Disadvantages Nonlinear Self heating Limited range 49 BME 301 Lecture Note 2 - Ali Işın 2017

50 Circuit Connections of Thermistors Bridge Connection to measure voltage R 1 R 3 V v a v b R 2 R t Amplifier Connection to measure currents 50 BME 301 Lecture Note 2 - Ali Işın 2017

51 Resistance ratio, R/R 25º C Thermistors Resistance Relationship between Resistance and Temperature at zero-power resistance of thermistor R t R 0 e [ ( T T ) / 0 TT 0 = material constant for thermistor, K (2500 to 5000 K) T o = standard reference temperature, K T o = K = 20C = 68F 1 R t drt dt T is a nonlinear function of temperature ] (% / 2 K ) (a) Typical thermistor zero-power resistance ratio-temperature characteristics for various materials. (a) Temperature, C 51 BME 301 Lecture Note 2 - Ali Işın 2017

52 Voltage, V Voltage-Versus-Current Characteristics The temperature of the thermistor is that of its surroundings. However, above specific current, current flow generates heat that make the temperature of the thermistor above the ambient temperature. B A C Air Water 1.0 (b) Current, ma (b) Thermistor voltage-versus-current characteristic for a thermistor in air and water. The diagonal lines with a positive slope give linear resistance values and show the degree of thermistor linearity at low currents. The intersection of the thermistor curves and the diagonal lines with the negative slope give the device power dissipation. Point A is the maximal current value for no appreciable self-heat. Point B is the peak voltage. Point C is the maximal safe continuous current in air. 52 BME 301 Lecture Note 2 - Ali Işın 2017

53 Radiation Thermometry The higher the temperature of a body the higher is the electromagnetic radiation (EM). Electromagnetic Radiation Transducers Convert energy in the form of EM radiation into an electrical current or potential,or modify an electrical current or potential. Medical thermometry maps the surface temperature of a body with a sensitivity of a few tenths of a Kelvin. Application Breast cancer, determining location and extent of arthritic disturbances, measure the depth of tissue destruction from frostbite and burns, detecting various peripheral circulatory disorders (venous thrombosis, carotid artery occlusions) 53 BME 301 Lecture Note 2 - Ali Işın 2017

54 54 BME 301 Lecture Note 2 - Ali Işın 2017

55 Radiation Thermometry Sources of EM radiation: Acceleration of charges can arise from thermal energy. Charges movement cause the radiation of EM waves. The amount of energy in a photon is inversely related to the wavelength: Thermal sources approximate ideal blackbody radiators: Blackbody radiator: 1 E 1 ev an object which absorbs all incident radiation, and emits the maximum possible thermal radiation (0.7 m to 1mm). J 55 BME 301 Lecture Note 2 - Ali Işın 2017

56 Spectral radient emittance, W-cm % Total power Power Emitted by a Blackbody Stefan-Boltzman law Power emitted at a specific wavelength: W 5 C C e 2 1 T 1 Unit : W/cm 2. m C 1 = 3.74x10 4 (W. m 4 /cm 2 ) C 2 = 1.44x10 4 (m. K) T = blackbody temperature, K = emissivity (ideal blackbody = 1) Wavelength for which W is maximum: m 2898 T m m varies inversely with T - Wien s displacement law BME 301 Lecture Note 2 - Ali Işın mm Wavelength, m m = 9.66 m T = 300 K % (a) Spectral radiant emittance versus wavelength for a blackbody at 300 K on the left vertical axis; percentage of total energy on the right vertical axis

57 Spectral radient emittance, W-cm % Total power Power Emitted by a Blackbody Stefan-Boltzman law W 5 C C e 2 1 T 1-2 mm m = 9.66 m 100% Total radiant power: T = 300 K 20 W t 2 W d T 1 Unit : W/cm 2. m 4 5 Wavelength, m % of the total radiant power is found in the wavelength band from 4 to 25 m 25 Stefan' s constant ( W / cm 2 ) K 4 57 BME 301 Lecture Note 2 - Ali Işın 2017

58 Thermal Detector Specifications Infrared Instrument Lens Properties; -pass wavelength > 1 m -high sensitivity to the weak radiated signal -Short response -Respond to large bandwidth Fused silica Sapphire Arsenic trisulfide Thallium bromide iodine Thermal Detectors -Low sensitivity -Respond to all wavelength Photon (Quantum) Detector -higher sensitivity -Respond to a limited wavelength Wavelength, m All thermal detectors 10 Fig. a 100 Fig. a) Spectral transmission for a number of optical materials. (b) Spectral sensitivity of photon and thermal detectors Wavelength, m Lead sulfide (PbS) 58 BME 301 Lecture Note 2 - Ali Işın Indium antimonide (InSb) (photovoltaic) Fig. b

59 Radiation Thermometer System Stationary chopped-beam radiation thermometer 59 BME 301 Lecture Note 2 - Ali Işın 2017

60 Application of Radiation Thermometer Measuring the core body temperature of the human by measuring the magnitude of infrared radiation emitted from the tympanic membrane and surrounding ear canal. Response time is 0.1 second Accuracy of 0.1 o C 60 BME 301 Lecture Note 2 - Ali Işın 2017

61 Fiber-Optic Temperature Sensors Small and compatible with biological implantation. Nonmetallic sensor so it is suitable for temperature measurements in a strong electromagnetic heating field. Gallium Arsenide (GaAs) semiconductor temperature probe. The amount of power absorbed increases with temperature 61 BME 301 Lecture Note 2 - Ali Işın 2017

62 Optical Measurements Applications: 1- Clinical-chemistry lab (analyze sample of blood and tissue) 2- Cardiac Catheterization (measure oxygen saturation of hemoglobin and cardiac output) Components: Sources, filters, and detectors. General block diagram of an optical instrument. (b) Highest efficiency is obtained by using an intense lamp, lenses to gather and focus the light on the sample in the cuvette, and a sensitive detector. (c) Solidstate lamps and detectors may simplify the system. 62 BME 301 Lecture Note 2 - Ali Işın 2017

63 Radiation Sources 1- Tungsten Lamps - Coiled filaments to increase emissivity and efficiency. - Ribbon filaments for uniform radiation - Tungsten-halogen lamps have iodine or bromine to maintain more than 90% of their initial radiant. Spectral characteristics of sources, (a) Light sources, Tungsten (W) at 3000 K has a broad spectral output. At 2000 K, output is lower at all wavelengths and peak output shifts to longer wavelengths. 63 BME 301 Lecture Note 2 - Ali Işın 2017

64 Radiation Sources 2- ARC Discharges - Low-pressure lamp: Fluorescent lamp filled with Argon-Mercury (Ar-Hg) mixture. Accelerated electron hit the mercury atom and cause the radiation of 250 nm (5 ev) wavelength which is absorbed by phosphor. Phosphor will emits light of longer visible wavelengths. - Fluorescent lamp has low radiant so it is not used for optical instrument, but can be turned on in 20 sec and used for tachistoscope to provide brief stimuli to the eye. - High pressure lamp: mercury, sodium, xenon lamps are compact and can provide high radiation per unit area. Used in optical instruments. 64 BME 301 Lecture Note 2 - Ali Işın 2017

65 Radiation Sources 3- Light-Emitting Diodes (LED) A p-n junction devices that are optimized to radiant output. -GaAs has a higher band gap and radiate at 900 nm. Switching time 10 nsec. -GaP LED has a band gap of 2.26 ev and radiate at 700 nm -GaAsP absorb two photons of 940 nm wavelength and emits one photon of 540 nm wavelength. Advantages of LED: compact, rugged, economical, and nearly monochromatic. Spectral characteristics of sources, (a) Lightemitting diodes yield a narrow spectral output with GaAs in the infrared, GaP in the red, and GaAsP in the green. 65 BME 301 Lecture Note 2 - Ali Işın 2017

66 Radiation Sources 4- Laser (Light Amplification by Stimulated Emission of Radiation) -He-Ne lasers operate at 633 nm with 100 mw power. -Argon laser operates at 515 nm with the highest continuous power level with 1-15 W power. -CO 2 lasers provide W of continuous wave output power. -Ruby laser is a solid state lasers operate in pulsed mode and provide 693 nm with 1-mJ energy. The most medical use of the laser is to mend tear in the retina. Spectral characteristics of sources, (a) Monochromatic outputs from common lasers are shown by dashed lines: Ar, 515 nm; HeNe, 633 nm; ruby, 693 nm; Nd, 1064 nm 66 BME 301 Lecture Note 2 - Ali Işın 2017

67 Optical Filters Optical filters are used to control the distribution of radiant power or wavelength. Power Filters Glass partially silvered: most of power are reflected Carbon particles suspended in plastic: most of power are absorbed Two Polaroid filters: transmit light of particular state of polarization Wavelength Filters Color Filters: colored glass transmit certain wavelengths Gelatin Filters: a thin film of organic dye dried on a glass (Kodak 87) or combining additives with glass when it is in molten state (corning 5-56 ). Interference Filters: Depositing a reflective stack of layers on both sides of a thicker spacer layer. LPF, BPF, HPF of bandwidth from 0.5 to 200nm. Diffraction grating Filters: produce a wavelength spectrum. 67 BME 301 Lecture Note 2 - Ali Işın 2017

68 Optical Filters Spectral characteristics of filters (b) Filters. A Corning 5-65 glass filter passes a blue wavelength band. A Kodak 87 gelatin filter passes infrared and blocks visible wavelengths. Germanium lenses pass long wavelengths that cannot be passed by glass. Hemoglobin Hb and oxyhemoglobin HbO pass equally at 805 nm and have maximal difference at 660 nm. Optical method for measuring fat in the body (fat absorption 930 nm Water absorption 970 nm 68 BME 301 Lecture Note 2 - Ali Işın 2017

69 Classifications of Radiation Sensors Thermal Sensors: absorbs radiation and change the temperature of the sensor. Change in output could be due to change in the ambient temperature or source temperature. Sensitivity does not change with wavelength Slow response Example: Pyroelectric sensor: absorbs radiation and convert it to heat which change the electric polarization of the crystals. Quantum Sensors: absorb energy from individual photons and use it to release electrons from the sensor material. sensitive over a restricted band of wavelength Fast response Less sensitive to ambient temperature Example: Eye, Phototube, photodiode, and photographic emulsion. 69 BME 301 Lecture Note 2 - Ali Işın 2017

70 Photoemissive Sensors Phototube: have photocathode coated with alkali metals. A radiation photon with energy cause electron to jump from cathode to anode. Photon energies below 1 ev are not large enough to overcome the work functions, so wavelength over 1200nm cannot be detected. Photomultiplier An incoming photon strikes the photocathode and liberates an electron. This electron is accelerated toward the first dynode, which is 100 V more positive than the cathode. The impact liberates several electrons by secondary emission. They are accelerated toward the second dynode, which is 100 V more positive than the first dynode, This electron multiplication continues until it reaches the anode, where currents of about 1 A flow through R L. Time response < 10 nsec 70 BME 301 Lecture Note 2 - Ali Işın 2017

71 Photoconductive Cells Photoresistors: a photosensitive crystalline materials such as cadmium Sulfide (CdS) or lead sulfide (PbS) is deposited on a ceramic substance. The resistance decrease of the ceramic material with input radiation. This is true if photons have enough energy to cause electron to move from the valence band to the conduction band. 71 BME 301 Lecture Note 2 - Ali Işın 2017

72 Photojunction Sensors Photojunction sensors are formed from p-n junctions and are usually made of silicon. If a photon has enough energy to jump the band gap, hole-electron pairs are produced that modify the junction characteristics. Photodiode: With reverse biasing, the reverse photocurrent increases linearly with an increase in radiation. Phototransistor: radiation generate base current which result in the generation of a large current flow from collector to emitter. Response time = 10 microsecond Voltage-current characteristics of irradiated silicon p-n junction. For 0 irradiance, both forward and reverse characteristics are normal. For 1 mw/cm 2, open-circuit voltage is 500 mv and short-circuit current is 8 A. 72 BME 301 Lecture Note 2 - Ali Işın 2017

73 Photovoltaic Sensors Photovoltaic sensors is a p-n junction where the voltage increases as the radiation increases. Spectral characteristics of detectors, (c) Detectors. The S4 response is a typical phototube response. The eye has a relatively narrow response, with colors indicated by VBGYOR. CdS plus a filter has a response that closely matches that of the eye. Si p-n junctions are widely used. PbS is a sensitive infrared detector. InSb is useful in far infrared. Note: These are only relative responses. Peak responses of different detectors differ by BME 301 Lecture Note 2 - Ali Işın 2017

74 Optical Combinations Total effective irradiance, is found by breaking up the spectral curves into many narrow bands and then multiplying each together and adding the resulting increments. E e S F D S = relative source output; F = relative filter transmission D = relative sensor responsivity Spectral characteristics of combinations thereof (d) Combination. Indicated curves from (a), (b), and (c) are multiplied at each wavelength to yield (d), which shows how well source, filter, and detector are matched. 74 BME 301 Lecture Note 2 - Ali Işın 2017

75 Measuring Core Temperature Because skin temperature cannot directly be correlated with interior body temperature, body (core) temperature measurement is traditionally performed inside a body cavity An old and traditional device used for body temperature measurement is the mercury thermometer that does not contain sensors. Its drawbacks are slow operation and difficult reading and registration of the result 75 BME 301 Lecture Note 2 - Ali Işın 2017

76 Electronic thermometers They generally contain diodes as temperaturesensing elements with a special package design that can assure small thermal capacity and good thermal conductivity to the environment. They have relatively short response times and good visible display units Structure of a disposable oral thermometer. 76 BME 301 Lecture Note 2 - Ali Işın 2017

77 Radiation Ear Thermometer This version is based on a pyroelectric sensor. Thermal radiation flux from the auditory canal is channeled by the optical waveguide toward the pyroelectric sensor. When pressing the start button, the shutter opens momentarily, exposing the sensor to thermal radiation and replacing the radiation coming from the shutter itself. An ambient temperature sensor element is behind the shutter. The radiation reaches the sensor where it is converted into electric current impulse due to the pyroelectric effect 77 BME 301 Lecture Note 2 - Ali Işın 2017

78 Skin Blood-Flow Sensor Skin blood flow (SBF) or skin perfusion is a complex phenomenon that occurs in capillaries. In perfused tissue, thermal conductivity depends not only on the thermal conductivity of the tissue materials, but also on the heat convection transferred by the blood flow in capillaries. Thus, thermal conductivity of the skin can vary within a wide range; its minimum value, 2.5 mw/cm C A Thermal Conductivity Sensor for the Measurement of Skin Blood Flow 78 BME 301 Lecture Note 2 - Ali Işın 2017

79 Sensors for Pressure Pulses and Movement Pulse sensing is a convenient and efficient way of acquiring important physiological information concerning the cardiovascular system. Finger pulse pickups can be employed in systems that measure blood pressure, heart rate, and blood flow 79 The pulse-wave signal is sent through the buffer to the signal-processing electronics. The PVDF film is in direct contact with the finger therefore, its metallized surfaces have to be shielded on both sides with thin metallized protecting polymer films and sealed with highly insulating silicone rubber to avoid damage to the surface electrodes BME 301 Lecture Note 2 - Ali Işın 2017

80 SENSORS IN ULTRASOUND IMAGING The first and simplest ultrasound imaging systems applied the A-mode (amplitude modulation) imaging illustrated in Figure 80 BME 301 Lecture Note 2 - Ali Işın 2017

81 ULTRASOUND IMAGING In B-mode (brightness modulation) imaging, all echo impulses are represented by a pixel on the display, and the brightness corresponds to the amplitude of the echo. To get a two-dimensional cross-sectional image, an appropriate scanning of the desired cross section is necessary Scanning methods in B-mode ultrasound imaging: (a) sequential linear array scanner, (b) mechanical sector scanner, and (c) phased array sector scanne 81 BME 301 Lecture Note 2 - Ali Işın 2017

82 The Doppler blood-flow measurement Doppler blood flow detectors operate by means of continuous sinusoidal excitation. The frequency difference calibrated for flow velocity can be displayed or transformed by a loudspeaker into an audio output. 82 BME 301 Lecture Note 2 - Ali Işın 2017

83 X-ray Imaging System In optically coupled CCD X-ray imaging system, X rays are impinged into a fluorescent screen and the image produced is then transferred onto the surface of an individual CCD by optical lenses 83 BME 301 Lecture Note 2 - Ali Işın 2017

84 Optical Coherence Tomography The technique of optical coherence tomography (OCT) provides a micronscale resolution crosssectional image from the overall eyeball, not only from the retina. OCT is similar to B-scan ultrasonic imaging Schematic diagram of optical coherence tomography instrumentation 84 BME 301 Lecture Note 2 - Ali Işın 2017